--- title: Migrator table_of_contents: true introduction: > The Sass migrator automatically updates your Sass files to help you move on to the latest and greatest version of the language. Each of its commands migrates a single feature, to give you as much control as possible over what you update and when. --- ## Usage To use the Sass migrator, tell it [which migration][] you want to run and what Sass files you want to migrate: [which migration]: #migrations ```shellsession sass-migrator ``` By default, the migrator will only change files that you explicitly pass on the command line. Passing the [`--migrate-deps` option][] tells the migrator to also change all the stylesheets that are loaded using the [`@use` rule][], [`@forward` rule][], or [`@import` rule][]. And if you want to do a test run to see what changes will be made without actually saving them, you can pass [--dry-run][] [--verbose][] (or `-nv` for short). [`--migrate-deps` option]: #migrate-deps [`@use` rule]: /documentation/at-rules/use [`@forward` rule]: /documentation/at-rules/forward [`@import` rule]: /documentation/at-rules/import [--dry-run]: #dry-run [--verbose]: #verbose {% render 'code_snippets/example-module-migrator' %} ## Installation You can install the Sass migrator from most of the same places that you can install [Dart Sass](/dart-sass): ### Standalone You can install the Sass migrator on Windows, Mac, or Linux by downloading the package for your operating system [from GitHub][] and [adding it to your `PATH`][]. [from GitHub]: {{ releases['migrator'].url }} [adding it to your `PATH`]: https://katiek2.github.io/path-doc/ ### npm If you use Node.js, you can also install the Sass migrator using [npm][] by running [npm]: https://www.npmjs.com ```shellsession npm install -g sass-migrator ``` ### Chocolatey If you use [the Chocolatey package manager][] for Windows, you can install the Sass migrator by running [the Chocolatey package manager]: https://chocolatey.org ```shellsession choco install sass-migrator ``` ### Homebrew If you use [the Homebrew package manager][] for Mac OS X, you can install Dart Sass by running [the Homebrew package manager]: https://brew.sh ```shellsession brew install sass/sass/migrator ``` ## Global Options These options are available for all migrators. ### `--migrate-deps` This option (abbreviated `-d`) tells the migrator to change not just the stylesheets that are explicitly passed on the command line, but also any stylesheets that they depend on using the [`@use` rule][], [`@forward` rule][], or [`@import` rule][]. ```shellsession $ sass-migrator module --verbose style.scss Migrating style.scss $ sass-migrator module --verbose --migrate-deps style.scss Migrating style.scss Migrating _theme.scss Migrating _fonts.scss Migrating _grid.scss ``` {% headsUp %} The [module migrator][] assumes that any stylesheet that is depended on using a [`@use` rule][] or a [`@forward` rule][] has already been migrated to the module system, so it won't attempt to migrate them, even when the `--migrate-deps` option is passed. [module migrator]: #module [`@use` rule]: /documentation/at-rules/use [`@forward` rule]: /documentation/at-rules/forward {% endheadsUp %} ### `--load-path` This option (abbreviated `-I`) tells the migrator a [load path][] where it should look for stylesheets. It can be passed multiple times to provide multiple load paths. Earlier load paths will take precedence over later ones. Dependencies loaded from load paths are assumed to be third-party libraries, so the migrator will not migrate them even when the [`--migrate-deps` option][] is passed. [load path]: /documentation/at-rules/use#load-paths ### `--dry-run` This flag (abbreviated `-n`) tells the migrator not to save any changes to disk. It instead prints the list of files that it would have changed. This is commonly paired with the [`--verbose` option][] to print the contents of the changes that would have been made as well. [`--verbose` option]: #verbose ```shellsession $ sass-migrator module --dry-run --migrate-deps style.scss Dry run. Logging migrated files instead of overwriting... style.scss _theme.scss _fonts.scss _grid.scss ``` #### `--no-unicode` This flag tells the Sass migrator only to emit ASCII characters to the terminal as part of error messages. By default, or if `--unicode` is passed, the migrator will emit non-ASCII characters for these messages. This flag does not affect the CSS output. ```shellsession $ sass-migrator --no-unicode module style.scss line 1, column 9 of style.scss: Error: Could not find Sass file at 'typography'. , 1 | @import "typography"; | ^^^^^^^^^^^^ ' Migration failed! $ sass-migrator --unicode module style.scss line 1, column 9 of style.scss: Error: Could not find Sass file at 'typography'. ╷ 1 │ @import "typography"; │ ^^^^^^^^^^^^ ╵ Migration failed! ``` ### `--verbose` This flag (abbreviated `-v`) tells the migrator to print extra information to the console. By default, it just prints the name of files that are changed, but when combined with the [`--dry-run` option][] it also prints those files' new contents. [`--dry-run` option]: #dry-run ```shellsession $ sass-migrator module --verbose --dry-run style.scss Dry run. Logging migrated files instead of overwriting... <==> style.scss @use "bootstrap" with ( $body-bg: #000, $body-color: #111 ); @include bootstrap.media-breakpoint-up(sm) { .navbar { display: block; } } $ sass-migrator module --verbose style.scss Migrating style.scss ``` ## Migrations ### Division This migration converts stylesheets that use [`/` as division] to use the built-in `math.div` function instead. [`/` as division]: /documentation/breaking-changes/slash-div #### `--pessimistic` By default, the migrator converts `/` operations to `math.div` even when it isn't sure that it will be division when evaluated. It only leaves them as-is when it can statically determine that they're doing something else (such as when there's no SassScript involved, or when one of the operands is a string). The `math.div` function currently functions identically to the `/` operator, so this is safe to do, but may result in new warnings if one of the arguments to `math.div` at runtime is not a number. If you want to avoid this behavior, you can pass the `--pessimistic` flag. With this flag, the migrator will only convert `/` operations that it knows for sure are doing division. This will prevent any unnecessary `math.div` conversions, but it's likely to leave some division unmigrated if it can't be statically determined. ### Module This migration converts stylesheets that use the old [`@import` rule][] to load dependencies so that they use the Sass module system via the [`@use` rule][] instead. It doesn't just naïvely change `@import`s to `@use`s—it updates stylesheets intelligently so that they keep working the same way they did before, including: - Adding namespaces to uses of members (variables, mixins, and functions) from other modules. - Adding new `@use` rules to stylesheets that were using members without importing them. - Converting overridden default variables to [`with` clauses][]. [`with` clauses]: /documentation/at-rules/use#configuration - Automatically removing `-` and `_` prefixes from members that are used from other files (because otherwise they'd be considered [private][] and could only be used in the module they're declared). [private]: /documentation/at-rules/use#private-members - Converting [nested imports][] to use the [`meta.load-css()` mixin][] instead. [nested imports]: /documentation/at-rules/import#nesting [`meta.load-css()` mixin]: /documentation/modules/meta#load-css {% headsUp %} Because the module migrator may need to modify both member definitions _and_ member names, it's important to either run it with the [`--migrate-deps` option][] or ensure that you pass it all the stylesheets in your package or application. [`--migrate-deps` option]: #migrate-deps {% endheadsUp %} {% render 'code_snippets/example-module-migrator' %} #### Loading Dependencies The module migrator needs to be able to read all of the stylesheets depended on by the ones it's migrating, even if the [`--migrate-deps` option][] is not passed. If the migrator fails to find a dependency, you'll get an error. ```shellsession $ ls . style.scss node_modules $ sass-migrator module style.scss Error: Could not find Sass file at 'dependency'. , 1 | @import "dependency"; | ^^^^^^^^^^^^ ' style.scss 1:9 root stylesheet Migration failed! $ sass-migrator --load-path node_modules module style.scss ``` If you use a [load path][] when compiling your stylesheets, make sure to pass that to the migrator using the [`--load-path` option][]. Unfortunately, the migrator does not support custom importers, but it does have built-in support for resolving URLs starting with `~` by searching in `node_modules`, similar to [what Webpack supports][]. [load path]: /documentation/at-rules/use#load-paths [`--load-path` option]: #load-path [what Webpack supports]: https://github.com/webpack-contrib/sass-loader#resolving-import-at-rules #### `--remove-prefix` This option (abbreviated `-p`) takes an identifier prefix to remove from the beginning of all variable, mixin, and function names when they're migrated. Members that don't start with this prefix will remain unchanged. The [`@import` rule][] put all top-level members in one global scope, so when it was the standard way of loading stylesheets, everyone was incentivized to add prefixes to all their member names to avoid accidentally redefining some other stylesheet's. The module system solves this problem, so it's useful to automatically strip those old prefixes now that they're unnecessary. ```shellsession $ cat style.scss @import "theme"; @mixin app-inverted { color: $app-bg-color; background-color: $app-color; } $ sass-migrator --migrate-deps module --remove-prefix=app- style.scss $ cat style.scss @import "theme"; @mixin inverted { color: theme.$bg-color; background-color: theme.$color; } ``` When you pass this option, the migrator will also generate an [import-only stylesheet][] that [forwards][] all the members with the prefix added back, to preserve backwards-compatibility for users who were importing the library. [import-only stylesheet]: /documentation/at-rules/import#import-only-files [forwards]: /documentation/at-rules/forward This option may be passed multiple times, or with multiple values separated by commas. Each prefix will be removed from any members that have it. If a member matches multiple prefixes, the longest matching prefix will be removed. #### `--forward` This option tells the migrator which members to forward using the [`@forward` rule][]. It supports the following settings: - `none` (the default) doesn't forward any members. - `all` forwards all members except those that started with `-` or `_` in the original stylesheet, since that was commonly used to mark a package-private member before the module system was introduced. - `prefixed` forwards only members that begin with the prefix passed to the [`--remove-prefix` option][]. This option may only be used in conjunction with the `--remove-prefix` option. [`--remove-prefix` option]: #remove-prefix All files that are passed explicitly on the command line will forward members that are transitively loaded by those files using the `@import` rule. Files loaded using the [`--migrate-deps` option][] will not forward any new members. This option is particularly useful when migrating a Sass library, because it ensures that users of that library will still be able to access all the members it defines. ```shellsession $ cat _index.scss @import "theme"; @import "typography"; @import "components"; $ sass-migrator --migrate-deps module --forward=all style.scss $ cat _index.scss @forward "theme"; @forward "typography"; @forward "components"; ``` ### Namespace This migration allows you to easily change the [namespaces][] of the `@use` rules in a stylesheet. This is useful if the namespaces that the module migrator generates to resolve conflicts are non-ideal, or if you don't want to use the default namespace that Sass determines based on the rule's URL. [namespaces]: /documentation/at-rules/use#choosing-a-namespace #### `--rename` You can tell the migrator which namespace(s) you want it to change by passing expressions to the `--rename` option. These expressions are of the form ` to ` or `url to `. In these expressions, `` and `` are [regular expressions][] which match against the entirety of the existing namespace or the `@use` rule's URL, respectively. [regular expressions]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions For simple use cases, this just looks like `--rename 'old to new'`, which would rename a `@use` rule with the namespace `old` to instead be `new`. However, you can also do this to accomplish more complicated renames. For instance, say that you previously had a stylesheet that looked like this: ```scss @import 'components/button/lib/mixins'; @import 'components/input/lib/mixins'; @import 'components/table/lib/mixins'; // ... ``` Since all of these URLs would have the default namespace `mixins` when migrated to `@use` rules, the module migrator may end up generating something like this: ```scss @use 'components/button/lib/mixins' as button-lib-mixins; @use 'components/input/lib/mixins' as input-lib-mixins; @use 'components/table/lib/mixins' as table-lib-mixins; // ... ``` This is valid code since the namespaces don't conflict, but they're way more complicated than they need to be. The relevant part of the URL is the component name, so we can use the namespace migrator to extract that part out. If we run the namespace migrator with `--rename 'url components/(\w+)/lib/mixins to \1'`, we'll end up with: ```scss @use 'components/button/lib/mixins' as button; @use 'components/input/lib/mixins' as input; @use 'components/table/lib/mixins' as table; // ... ``` The rename script here says to find all of the `@use` rules whose URLs look like `components/(\w+)/lib/mixins` (`\w+` in a regular expression means to match any word of one or more characters). The `\1` in the output clause means to substitute in the contents of the first set of parentheses in the regular expression (called a [group][]). [group]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Groups_and_Ranges If you wish to apply multiple renames, you can pass the `--rename` option multiple times, or separate them with a semicolon or a line break. Only the first rename that applies to a given rule will be used, so you could pass something like `--rename 'a to b; b to a'` to swap the namespaces `a` and `b`. #### `--force` By default, if two or more `@use` rules have the same namespace after the migration, the migrator will fail, and no changes will be made. In this case, you'll usually want to adjust your `--rename` script to avoid creating conflicts, but if you'd prefer to force the migration, you can instead pass `--force`. With `--force`, if any conflicts are encountered, the first `@use` rule will get its preferred namespace, while subsequent `@use` rules with the same preferred namespace will instead have a numerical suffix added to them.